No need for dark energy , gravity will suffice.

[PeopleSmoks]

Yes.


Well, there is some slight uncertainty in redshift due to the peculiar velocity of the source object (an example of a "peculiar velocity" would be a galaxy falling into a massive cluster). But that's about it. Gravity doesn't affect the redshift of astronomical objects by any significant amount.

How does gravitational lensing affect redshift? And not to make too many assumptions, but when looking over great distances, I personally, would assume that this effect might occur several times before the light of a very distant objects reach our viewpoint. If Lambda is a very small number, and gravitational affects on redshift are very small, wouldn't there be an accumulative effect over great distances?

If we are to say that bodies in the cosmos aren't affected by gravitation because they reach terminal velocity, how are we measuring the velocity of an object moving away from us if as you say there is some slight uncertainty?

Every answer seems to bring more questions. I guess that's called learning and maybe thinking? Thanks for answering.

 

[Chalnoth]

How does gravitational lensing affect redshift?

Gravitational lensing has no effect upon redshift. It does have an effect upon brightness, however, and this does need to be taken into account when there is a massive object almost directly along the line of sight, which does happen. Most of the time it's not a significant effect.

If Lambda is a very small number, and gravitational affects on redshift are very small, wouldn't there be an accumulative effect over great distances?

Well, right, but the effect of the cosmological constant is to make distant objects dimmer than we would expect if there were no such thing, while the effect of gravitational lensing, if not accounted for, is to make distant objects brighter than we would expect (and in a very erratic way, depending upon what lies along the line of sight).

If we are to say that bodies in the cosmos aren't affected by gravitation because they reach terminal velocity, how are we measuring the velocity of an object moving away from us if as you say there is some slight uncertainty?

Er, I don't know what you mean by terminal velocity, but what we do measure is the redshift, which is a measure of how much the universe has expanded since the light was emitted, and the distance (through a variety of different methods). Measuring the redshift and distance of many different objects in the universe gives us a measure of the history of the expansion of the universe. It's not actually accurate to say that we're measuring the velocity of far-away objects, because in strict terms, the velocity of a distant object is meaningless. Relative velocity between two objects can, in General Relativity, only be determined for two objects at the same location.

Every answer seems to bring more questions. I guess that's called learning and maybe thinking? Thanks for answering.

Indeed.

 

[PeopleSmoks]

Er, I don't know what you mean by terminal velocity, but what we do measure is the redshift, which is a measure of how much the universe has expanded since the light was emitted, and the distance (through a variety of different methods). Measuring the redshift and distance of many different objects in the universe gives us a measure of the history of the expansion of the universe. It's not actually accurate to say that we're measuring the velocity of far-away objects, because in strict terms, the velocity of a distant object is meaningless. Relative velocity between two objects can, in General Relativity, only be determined for two objects at the same location.

I meant escape velocity. Terminal velocity is something completely different. Does that change anything you wrote? What I'm trying to grasp is just how is it concluded that cosmic expansion is accelerating? What is used to measure the rate of expansion?

 

[Chalnoth]

I meant escape velocity. Terminal velocity is something completely different. Does that change anything you wrote? What I'm trying to grasp is just how is it concluded that cosmic expansion is accelerating? What is used to measure the rate of expansion?

The original evidence came in from observations of distant supernovae, back in 1999:
http://adsabs.harvard.edu/abs/1999ApJ...517..565P

The basic idea is that they measured the relationship between redshift and brightness of many supernovae. General Relativity predicts a very specific relationship between the two. From the redshift of the supernova, we compute the luminosity distance:

$$D_L = c(1+z)\int_0^z \frac{dz}{H(z)}$$

Here $$D_L$$ is the luminosity distance, $$z$$ is the redshift of the object, $$c$$ is the speed of light, and $$H(z)$$ is the Hubble parameter as a function of redshift, as given by the Friedmann equation.

The luminosity distance is constructed in such a way that the observed brightness of an object falls off as $$1/D_L^2$$, so that it matches up to the brightness falloff we see for local objects. The equation can be derived directly from the FRW metric.

Once we have the luminosity distance, we have to determine how it effects the brightness of the object as observed. Objects in astronomy have their brightness measured in terms of their magnitude. The magnitude of an object $$m$$ is defined as follows:

$$m = M + 5\left(\log_{10}D_L - 1\right)$$

Here $$m$$ is the magnitude we observe from Earth, and $$M$$ is the intrinsic magnitude of the objects.

Since these supernovae are all nearly the same brightness, we can use these as "standard candles": with some uncertainty, $$M$$ is the same for all of this particular type of supernova. So we can use a large number of supernovae to get an estimate of the luminosity distance $$D_L$$ for each of them. Since we also have a redshift for each of these supernovae, we get an estimate of the function $$H(z)$$, which is the expansion history. From this measure, we find that the way $$H(z)$$ has changed through time indicates that in recent times, the second derivative of the scale factor with respect to time, $$\ddot{a}$$, has become positive.

Did that help?

 

[PeopleSmoks]

Thanks, and also thanks for the Harvard link. I'll be taking some time to better absorb the information as well as look up more articles etc. I'm sure it will only lead to a few more questions, but hopefully I will pose them based on fewer misconceptions.

 

[Peter Watkins]

Thank you for your replies, but this time you've got it wrong. The scenario described in reply #26 applies to on object escaping from a large body such as a planet or star which has a fixed measurement of gravity at anyone particular place, which lessens as the object moves away and leaves the gravity source behind. The expanding universe was entirely different in that it was escaping from itself. With each doubling of the diameter, and these were frequent and fast in the early days, the gravity at anyone point was reduced by a factor of eight. Additionally, the gravity wasn't left behind as in escape from a fixed source, but instead accompanied the outward moving matter and thus was, and is, able to maintain it's advantage in the gravity/kinetic energy battle. You are correct in stating that gravity can be overcome temporarily, a tot can throw a rattle upwards, but not with sufficient energy to escape. And this is state that the universe is in, temporary escape. You will, of course, refer to the increased rate at which galaxies are moving apart, but this is indicative of a slowing in the rate of expansion.
And I would state again that if escape velocity had been reached, gravity would have had no effect. In short, the detectable presence of gravity = collapse. It really is that simple.

 

[Chalnoth]

And I would state again that if escape velocity had been reached, gravity would have had no effect. In short, the detectable presence of gravity = collapse. It really is that simple.

No. It isn't. If you have escape velocity, gravity still has an effect: it slows you down, and, depending upon the motion, deflects that path. If your statement were true, after all, it would mean that gravity can have no effect upon light, because light always has escape velocity (except with a black hole, of course).

 

[PeopleSmoks]

What I am beginning to see is that the "pivot" point in this is dark energy. You see, I am trying to absorb all the information, but I still look at the manner in which the numbers are used in various equations to concur with various theory. My first example of this is Einstein's cosmological constant, which originally confirmed a static universe, until Hubble's obeservations showed expansion. Then, the numbers were modified to coincide with the new data.

Chalnoth is telling us that gravity continues to have an affect, despite excape velocity being obtained. Yes, I understand this. So at what point or what distance would be required for an object to actually escape gravity and it's effects? I also understand that this is relative to the mass of both objects, but what equation proves it and does this equation require dark energy as a positive to make it work?

If positive dark energy is used in this equation, this again brings me full circle in a way as to wondering why dark energy is given a positive value? I read an article that stated dark matter contains equal positive and negative particles, which should make it neutral. Isn't dark energy related to dark matter?

 

[Chalnoth]

Chalnoth is telling us that gravity continues to have an affect, despite excape velocity being obtained. Yes, I understand this. So at what point or what distance would be required for an object to actually escape gravity and it's effects?

You can't. Gravity is an infinite-range force. What is meant by "escape velocity" is not that it escapes the effects of gravity, but rather that the object can escape off to infinity (in infinite time). It will still feel the gravity for its entire travel time.

Gravity is termed an "infinite range force" because it falls off at 1/r^2. By contrast, magnetism is not infinite range because it falls off at 1/r^3 (as there are no magnetic monopoles). And while the range of the electric force is technically infinite as well, as it also falls off as 1/r^2, in reality at large distances it never falls off slower than 1/r^3, because electric charges don't build up in one place due to the strength of the force (so you don't get electric charges building up in anyone area). So gravity is, in effect, the only infinite range force around.

I also understand that this is relative to the mass of both objects, but what equation proves it and does this equation require dark energy as a positive to make it work?

Actually I'd have to go back and look up precisely why this is the case, but I remember that the fundamental quality that makes a force infinite in range is that it falls off at 1/r^2. I could probably figure it out if I thought about it, but I'm too tired for that right now.

If positive dark energy is used in this equation, this again brings me full circle in a way as to wondering why dark energy is given a positive value? I read an article that stated dark matter contains equal positive and negative particles, which should make it neutral. Isn't dark energy related to dark matter?

Well, dark matter is considered very likely to consist of both matter and anti-matter due to its weak interactions (the weak interactions mean it hasn't yet had time for all the dark matter particles to annihilate with their anti-particles). It's all neutrally-charged, though: it has to be, or else it wouldn't be dark.

As for dark energy, we don't know. There are some theorists who have come up with theories that attempt to place dark energy and dark matter into the same framework. I think it'd be neat if one of those theories turned out to be accurate, but I strongly suspect that nothing of the sort is true. The positive value, by the way, is what fits the observed expansion. A negative value would tend to cause the universe to recollapse, not accelerate its expansion.

 

[PeopleSmoks]

Chalnoth, not looking for an answer tonight, but a few more questions. Again, articles I've read, don't refer to dark matter particles as being matter and antimatter, but simply as positively and negatively charged subatomic particles. It was further described as being in a plasma state as opposed to gasious. Is it possible that dark matter is the result of dark energy and not the other way? Kind of switching around E*=*mc2, where mass is created by dividing energy by the speed of light?

Also, once again, not to place a negative value to dark energy, but possibly a neutral or inert energy, how would that affect cosmic expasion? Would this permit the eventual deceleration and future collapse of the universe without undermining the facts of present expansion?

One last thing. Could the acceleration of cosmic expansion and the energy levels be compared to the expansion and energy release of a dying star throwing off hydrogen before it's collapse?

 

[Chalnoth]

Chalnoth, not looking for an answer tonight, but a few more questions. Again, articles I've read, don't refer to dark matter particles as being matter and antimatter, but simply as positively and negatively charged subatomic particles. It was further described as being in a plasma state as opposed to gasious.

Oh, well that's either a misunderstanding, or those articles you've read are entirely wrong. If the dark matter were charged, it wouldn't be dark.

Is it possible that dark matter is the result of dark energy and not the other way? Kind of switching around E*=*mc2, where mass is created by dividing energy by the speed of light?

Dark matter is most likely a result of thermal production in the early universe. That is, when the temperature was high enough, the forces by which the dark matter particles interact were active. When the temperature cooled to the point that those forces were no longer active, the dark matter just stuck around. Due to other constraints (particularly big bang nucleosynthesis), at the time dark energy can't actually have had much effect on the universe.

Also, once again, not to place a negative value to dark energy, but possibly a neutral or inert energy, how would that affect cosmic expasion? Would this permit the eventual deceleration and future collapse of the universe without undermining the facts of present expansion?

Well, it's the interaction with gravity that determines how dark energy affects the expansion. Basically that interaction would have to turn off at some point in the future, such as through the decay of the field that makes up dark energy. And then it would have to turn out that the universe had a positive curvature. The probability that both are the case is, frankly, pretty low, given current observation.

One last thing. Could the acceleration of cosmic expansion and the energy levels be compared to the expansion and energy release of a dying star throwing off hydrogen before it's collapse?

I don't see how they would be in any way related.

 

[Peter Watkins]

Chalnoth, thank you for your time again, but I think that your reply once again refers to escape from a solid body. I doubt that the astronauts were slowed as they departed for the moon. I know that they had a rocket motor to propel them out of orbit, but this only burned for a very short time. And as they weren't headed toward the moon but rather the place where the moon would be, there was no assistance there. There was a probe whose motor malfunctioned and this was placed into an elliptical orbit in order to "wind it up". When this was finally "slung out" I doubt that gravity slowed that either. When I said "no effect whatsoever" I meant that no matter, planets, stars etc. would have formed, so there would have been no light to be deflected.

 

[PeopleSmoks]

Chalnoth, thank you for your time again, but I think that your reply once again refers to escape from a solid body.

I see your point, but as I am trying to understand what Chalnoth is saying about gravitational effects, I think that at great distances, it is like a fly on an elephants back. The weight of the fly is there, but so small that it's affects on the elephant are negligable. Still, I wonder if there is any data showing orbital wobble of venus or Mars when their orbits align with earth.

Oh, well that's either a misunderstanding, or those articles you've read are entirely wrong. If the dark matter were charged, it wouldn't be dark.

I never said that dark matter was charged, but that the particles are, and being of equal number and mass that would mean that dark matter must be a neutral plasma.

Dark matter is most likely a result of thermal production in the early universe. That is, when the temperature was high enough, the forces by which the dark matter particles interact were active. When the temperature cooled to the point that those forces were no longer active, the dark matter just stuck around. Due to other constraints (particularly big bang nucleosynthesis), at the time dark energy can't actually have had much effect on the universe.

You see, this suggests that dark matter was created during the big bang. But, to maintain the balance of mass in the universe as it is now expanding, doesn't more dark matter have to constantly be created? If there is no 'new' dark matter being created, and we have enough today to fill the universe, we would have had too much yesterday, and not enough to fill the universe tomorrow.

 

[Chalnoth]

Actually I'd have to go back and look up precisely why this is the case, but I remember that the fundamental quality that makes a force infinite in range is that it falls off at 1/r^2. I could probably figure it out if I thought about it, but I'm too tired for that right now.

Just fyi, I thought about it some more, and this is the reason:

An infinite range force is one in which Gauss's Law holds. The idea of Gauss's Law with gravity is that when you are some distance away from the center of mass of a source, the force doesn't actually depend upon what the distribution of that mass is, provided it's spherical. That is, it doesn't matter how dense it is. This has a rather neat geometric description in that if you imagine a shell that completely encloses the object, the total flux of the force field through that shell will always be the same, no matter how big that shell is.

By contrast, if you take a force that falls off as 1/r^3 (or any faster than 1/r^2), then the bigger your shell is, the smaller the flux of the force field through that shell. This indicates that the total effects of the force fall off with distance, whereas with 1/r^2 the total effects extend out to infinity.

 

[Chalnoth]

Chalnoth, thank you for your time again, but I think that your reply once again refers to escape from a solid body. I doubt that the astronauts were slowed as they departed for the moon. I know that they had a rocket motor to propel them out of orbit, but this only burned for a very short time.

Of course they were. They just obtained escape velocity (or near enough to it to get as far as the moon, anyway). They had to be slowed by gravity because they kept gaining potential energy as they got further from the Earth. It's just that they had enough velocity once the rocket burn ended to still make the trip.

Think of it like coasting on a car: if you start off with enough speed, you can pop the car in neutral and still get a fair distance. This doesn't mean that friction doesn't continue to slow the car: it does. It just takes a little bit of time to stop it. And if you had enough velocity to start with, you could reach your destination (whatever that might be).

 

[Chalnoth]

I never said that dark matter was charged, but that the particles are, and being of equal number and mass that would mean that dark matter must be a neutral plasma.

No, because in a plasma, the positively and negatively charged particles are separated, so plasmas tend to glow pretty brightly depending upon their temperatures. This is the reason the Sun glows, for instance. It's also the reason why we see galaxy clusters in the X-ray range: they have a plasma trapped in their gravity well that is so hot it emits in the X-ray.

You see, this suggests that dark matter was created during the big bang. But, to maintain the balance of mass in the universe as it is now expanding, doesn't more dark matter have to constantly be created?

No. The universe is just getting less dense. And it has to have formed in the very early universe, because we see its effects as early as the cosmic microwave background.

If there is no 'new' dark matter being created, and we have enough today to fill the universe, we would have had too much yesterday, and not enough to fill the universe tomorrow.

Are you somehow thinking about this curvature issue? The open/closed/flat stuff? That doesn't change with time. The density changes, but the expansion rate changes to compensate. So the overall spatial curvature doesn't change.

 

[PeopleSmoks]

Of course they were. They just obtained escape velocity (or near enough to it to get as far as the moon, anyway). They had to be slowed by gravity because they kept gaining potential energy as they got further from the Earth. It's just that they had enough velocity once the rocket burn ended to still make the trip.

Think of it like coasting on a car: if you start off with enough speed, you can pop the car in neutral and still get a fair distance. This doesn't mean that friction doesn't continue to slow the car: it does. It just takes a little bit of time to stop it. And if you had enough velocity to start with, you could reach your destination (whatever that might be).

It seems a more simplistic way to look at the Earth's gravity as having an affect on travel to the moon is the moon's orbit. It's the combination of the moon's orbital speed and the Earth's gravitational effect that holds the moon in it's present orbit. Although, I have read that the moon's orbit is increasing. I read that the Earth's gravitational effect reaches as far as the Kuiper belt. I find there is may be some confusion between escaping Earth's gravity and escaping the atmosphere.

No, because in a plasma, the positively and negatively charged particles are separated, so plasmas tend to glow pretty brightly depending upon their temperatures. This is the reason the Sun glows, for instance. It's also the reason why we see galaxy clusters in the X-ray range: they have a plasma trapped in their gravity well that is so hot it emits in the X-ray.

There are a few different kinds of plasma that I have read about, including the polar auroras, but I also read that plasma is the base of intergalactic medium, and that not all plasma glows. As you have said, dark matter is called "dark" because we don't know it's true nature. It's also possible that if dark matter glows in some manner, it is beyond the infrared or ultraviolet scales.

No. The universe is just getting less dense. And it has to have formed in the very early universe, because we see its effects as early as the cosmic microwave background.


Are you somehow thinking about this curvature issue? The open/closed/flat stuff? That doesn't change with time. The density changes, but the expansion rate changes to compensate. So the overall spatial curvature doesn't change.

I think I'm beginning to see my misunderstandings regarding the differences between mass of the universe and it's volume density. Which also brings up the question, if the density of the mass is decreased, won't it's gravity decrease proportionately? Remember, we're talking about dark matter being spread across the cosmos and not a star.

 

[Chalnoth]

It seems a more simplistic way to look at the Earth's gravity as having an affect on travel to the moon is the moon's orbit. It's the combination of the moon's orbital speed and the Earth's gravitational effect that holds the moon in it's present orbit. Although, I have read that the moon's orbit is increasing.

The reason the moon's orbit is increasing is basically because the Earth isn't a perfect solid. Because the moon slightly deforms the shape of the Earth (mostly by causing the tides), and because the rotational speed of the Earth is different from the orbital speed of the moon, this causes an additional tidal force between the Earth and the moon. This tidal force has the effect of slowing down the Earth's rotation while at the same time increasing the moon's orbit, and will continue until the Earth and moon always have the same side facing one another.

I read that the Earth's gravitational effect reaches as far as the Kuiper belt.

It's infinite. Gravity is an infinite range force. Now, if you're far enough away from the solar system, you won't be able to distinguish between the Sun's mass and that of the Earth (or other planets, mostly Jupiter). But the amount of mass will still be important, and the Earth makes up some tiny fraction of the mass of the solar system.

I find there is may be some confusion between escaping Earth's gravity and escaping the atmosphere.

Possibly. But I think it's more a confusion between being able to escape Earth's gravity and simply not feeling the effects of it at all. If you have enough velocity, the Earth's gravity won't ever be enough to turn you around. But it still slows you down, the entire way.

There are a few different kinds of plasma that I have read about, including the polar auroras, but I also read that plasma is the base of intergalactic medium, and that not all plasma glows. As you have said, dark matter is called "dark" because we don't know it's true nature. It's also possible that if dark matter glows in some manner, it is beyond the infrared or ultraviolet scales.

The intergalactic medium is made up of normal matter. The word is not used, so far as I am aware, to talk about the much more abundant dark matter. And all plasma most definitely glows, at a range of wavelengths determined by its temperature.

Finally, as far as dark matter emitting in different wavelengths, that's not really possible because then dark matter would interact just as strongly as normal matter, and it wouldn't form dramatically different mass distributions than the normal matter (dark matter tends to be broadly-distributed, while normal matter tends to be stuck in tight clumps).

I think I'm beginning to see my misunderstandings regarding the differences between mass of the universe and it's volume density. Which also brings up the question, if the density of the mass is decreased, won't it's gravity decrease proportionately? Remember, we're talking about dark matter being spread across the cosmos and not a star.

Well, in a way this is so. As the normal and dark matter get spread more thinly, their effect on the rate of expansion diminishes.

 

[Peter Watkins]

Chalnoth, I suppose that what you're saying is pretty much what I was saying, ie. gravity is slowing expansion.

 

[Chalnoth]

Chalnoth, I suppose that what you're saying is pretty much what I was saying, ie. gravity is slowing expansion.

That's only how it works with normal matter. And even if all we had was normal matter, it's still possible for it to never slow expansion enough that it stops and recollapses.

 

[Jorrie]

Are you somehow thinking about this curvature issue? The open/closed/flat stuff? That doesn't change with time. The density changes, but the expansion rate changes to compensate. So the overall spatial curvature doesn't change.

Isn't it so that any spatial curvature present (not precisely flat) will evolve away from flatness as time goes on? It is true that open will always remain open and closed will always remain closed, but not at the same $\Omega$, not so?

 

[Chalnoth]

Isn't it so that any spatial curvature present (not precisely flat) will evolve away from flatness as time goes on? It is true that open will always remain open and closed will always remain closed, but not at the same $\Omega$, not so?

Well, it does depend upon how you write down the parameters. Take a look at the Friedmann equation with curvature:

$$H^2(a) = H^2_0 \left(\frac{\Omega_m}{a^3} + \frac{\Omega_r}{a^4} + \frac{\Omega_k}{a^2} + \Omega_\Lambda \right)$$

Written this way, the curvature parameter $$\Omega_k$$ does not change with a. You can think of this as saying that there's still the same total amount of "k" around, just as there's still the same amount of matter around, no matter how much the universe expands. However, if you look at how the effect of the curvature changes with the scale factor a, it dilutes more slowly than normal matter/dark matter or radiation. This means that even if the density fraction in the early universe was small, it tends to grow very rapidly compared to matter and radiation.

However, if there is a cosmological constant, that doesn't dilute at all, which means that the eventually any amount of cosmological constant will cause the curvature to also dilute away to nothing, leaving the dominant effect the cosmological constant at late times. If there was no cosmological constant (or anything else that acts like one), at late times the curvature would become dominant, as long as it is even slightly non-zero.

This is actually why inflation explains why the curvature was so small in our early universe: inflation behaved very much like a cosmological constant, and so it diluted the curvature away to nearly zero before our region began.

 

[Jorrie]

Well, it does depend upon how you write down the parameters. Take a look at the Friedmann equation with curvature:

$$H^2(a) = H^2_0 \left(\frac{\Omega_m}{a^3} + \frac{\Omega_r}{a^4} + \frac{\Omega_k}{a^2} + \Omega_\Lambda \right)$$

Written this way, the curvature parameter $$\Omega_k$$ does not change with a. You can think of this as saying that there's still the same total amount of "k" around, just as there's still the same amount of matter around, no matter how much the universe expands. However, if you look at how the effect of the curvature changes with the scale factor a, it dilutes more slowly than normal matter/dark matter or radiation...

Thx, that's the way I understood it too: $$\Omega_k$$ is the present curvature parameter and the $$a^2$$ denominator sorts out the evolution of the curvature contribution, if nonzero to start with. But as you said, a dominant $$\Omega_\Lambda$$ can also drive nonzero curvature to zero, so it is not always driven farther away from zero, as I implied...

 

[Naty1]

String theory appears to be the only thing that seems to completely exclude the possibility of cosmic collapse or the cyclic model.

Not sot: Heterotic M theory (Horava and Witten) with two parallel branes leads to the ekpyrotic cyclical model proposed by Paul Steinhardt and Neil Turok and explained in the book the Endless Universe, 2007. I'm reading it now and the main idea is that in the last ten years major ideas have emerged to revive cyclic models, a major one being that dark energy evolves over time. It speeds up the expansion rate, as is now being observed, acts as a shock absorber of the type used on automatic door closers, and also slowly decays over time shutting itself off.
Dark energy is not a constant!

 

[Peter Watkins]

Dark energy could only qualify as a cosmological constant if it existed, and this is very doubtful. When described it is always as an expanding force between galaxies, pushing them apart, much as expanding space is described. But what about galaxies between opposing forces? Picture a single galaxy surrounded by dark energy. Pushed from all sides it would go nowhere. The same would apply equally to a cluster of galaxies or a whole universe of galaxies. The overall effect would be neutral, as it would be with vacuum energy, an earlier attempt to explain galactic movement.

 

[Chalnoth]

Dark energy could only qualify as a cosmological constant if it existed, and this is very doubtful. When described it is always as an expanding force between galaxies, pushing them apart, much as expanding space is described. But what about galaxies between opposing forces? Picture a single galaxy surrounded by dark energy. Pushed from all sides it would go nowhere. The same would apply equally to a cluster of galaxies or a whole universe of galaxies. The overall effect would be neutral, as it would be with vacuum energy, an earlier attempt to explain galactic movement.

By this same logic, nothing could speed up or slow down the expansion. After all, being pushed equally in all directions is identical, in your little thought experiment, to being pulled in all directions. But the expansion rate has changed over time, so obviously something is wrong with your little thought experiment.

 

[Peter Watkins]

Hello Chalnoth. Re above; it is not a thought experiment, it is a simple statement of fact. Cosmologists seem to have forgotten that there is a fundamental law that applies throughout the universe ie., for every action there is an equal and opposite reaction. With regard to nothing to speed up expansion, you are quite right. It is an impossibility. But the rate at which galaxies move apart will increase as the expansion rate is slowed from within. This would resemble acceleration from our viewpoint. You yourself went to great lengths to impress upon me that gravity will always exert a restraining effect, and indeed it is, slowing the rate at which the universe is expanding. It can also cause acceleration, after first slowing the innermost galaxies to a halt, and then pulling them back at an increasing rate. If the universe was made up of layers, like an onion, it is these that are being separated. The outer layers, including us, are still moving outward at a rate that decreases toward the centre, whilst the inner layers are collapsing inward at a rate that increases toward the centre. This picture fits all observations. So, to state that there is no mechanisms to alter expansion rates is incorrect.

 

[Chalnoth]

So, to state that there is no mechanisms to alter expansion rates is incorrect.

You seem to be very confused. The FRW universe is one wherein the universe is modeled as a perfectly uniform fluid. This is known to be not entirely accurate, but works pretty well on large scales. And yes, a perfectly uniform fluid can expand and contract based upon the action of gravity combined with whatever internal pressures this fluid feels.

You can argue in words all you like, but the mathematics is unambiguous and certain: gravity does propel a changing rate of expansion.

 

[Peter Watkins]

Why on Earth is the universe modeled as a perfectly formed fluid? I would have thought that a perfectly formed vacuum would be more appropriate. I agree that gravity can alter he rate of expansion, but only by slowing. Cosmology is a science of observation and reason, not mathematics. I'll wager you that mathematics could not begin to describe our solar system to the un-informed, whilst words and illustrations would quickly convey the message.
Believe me, there is no confusion on my part. I thank you for your time.

 

[Chalnoth]

Why on Earth is the universe modeled as a perfectly formed fluid?

Not perfectly formed. Perfectly uniform. That's the FRW universe, anyway. What cosmologists typically work with is perturbed FRW, where that assumption of uniformity is relaxed.

But regardless, the reason why it's modeled that way is because it's a good approximation.

I agree that gravity can alter he rate of expansion, but only by slowing.

Well, you'd be wrong. It depends upon the matter/energy content of the universe.